As known, the bicycle is a mechanical means of transport moved by muscle force that is transmitted to the rear “driving” wheel through a motion transmission system. It comprises a pair of cranks, on which the cyclist exerts a propulsive thrust, one or more driving gear wheels, set in rotation by direct coupling with the pedal cranks, and one or more driven gear wheels or “sprockets”, set in rotation by the driving gear wheels through a chain, said sprockets being coupled with the hub of the rear wheel.
In particular, racing bicycles comprise a plurality of sprockets of various diameters and a plurality of driving gear wheels, also of various diameters. The chain simultaneously engages a driving gear wheel and a sprocket and can selectively move on them through a front gearshift and a rear gearshift, so as to obtain the combination of driving gear wheel and sprocket that offers the transmission ratio most suited to the travel conditions.
The front gearshift is mounted on the bicycle frame close to the plurality of driving gear wheels and comprises a derailleur that moves the chain from one driving gear wheel to another. The rear gearshift is mounted on the bicycle frame close to the plurality of sprockets and comprises a derailleur that moves the chain from one sprocket to another.
In the prior art, derailleurs of the front and rear gearshifts are made according to an articulated parallelogram mechanism in which the sides of the parallelogram are articulated together in pairs, along respective substantially parallel rotation axes, through pins. In particular, such an articulated parallelogram is formed from a support body, intended to stay fixed with respect to the frame, a so-called chain guide suitable for moving the chain between different engagement positions on respective gear wheels (the gears of the crankset, in the case of the derailleur of the front gearshift, or the sprockets of the sprocket set, in the case of the derailleur of the rear gearshift), and a pair of articulation arms or connecting rods (generally identified as outer connecting rod and inner connecting rod), which connect the chain guide to the support body in a mobile manner. In the case of the derailleur of the front gearshift, the chain guide comprises the element that in the jargon is called “fork”, whereas in the case of the derailleur of the rear gearshift, the chain guide comprises a rocker arm.
Each connecting rod is rotatably connected to the support body and to the chain guide through respective articulation pins inserted in respective holes.
The derailleurs can be manually actuated (such derailleurs are also known as mechanical derailleurs) or motor actuated (such derailleurs are also known as electrically driven derailleurs).